The screw and nut is one of the classical mechanical machines. In one typical use of the screw and nut machine, the screw is rotated by an external driving means, such as a motor, about its axis. The rotation of the screw then drives the nut axially, that is, parallel to the axis of the screw. By affixing another device to the nut, the rotational movement of the motor is transformed to lateral movement for that other device.
In its simplest form, the screw and nut machine makes use of the contact between matching helical threads on both the screw and the nut to cause the transformation of rotational movement to axial movement. This contact, however, creates a great amount of friction through which energy is lost. In addition, considerable heat may be generated to destroy the mechanisms under certain circumstances. To overcome this friction, people long ago suggested replacing the threads of the nut with helical tracks of bearing balls which roll along the rotating screw, and thereby substantially reduce the amount of friction between the screw and nut.